This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application for a patent entitled Device And Method For Generating Spread Spectrum Signals earlier filed on Dec. 31, 1997 in the Korean Industrial Property Office and there duly assigned Serial No. 80590/1997.
1. Field of the Invention
The present invention relates to a direct sequence-code division multiple access (i.e., DS-CDMA) mobile communication process and system, and, more particularly, to an offset quadrature phase shift keying (i.e., OQPSK) modulation device and process for generating spread spectrum signals using orthogonal codes.
2. Description of the Related Art
A code division multiple access (i.e., a CDMA) mobile communication system allows many subscribers to share time and frequency. To do so, a transmitter spreads signals is to be transmitted using pseudo-noise (i.e., PN) sequences (or PN codes), having low mutual correlation, allocated to the respective subscribers, and then transmits the spread signals. A receiver then generates the same PN sequences as used in the transmitter to maintain synchronization and despreads the received signals using the generated PN sequences to restore the original signals. An exemplar of one technique for despreading may be found in U.S. Pat. No. 5,577,025 for Signal Acquisition In A Multi-user Communication System Using Multiple Walsh Channels by Gordon Skinner and Brian Harms.
The data rate used by one subscriber is generally much lower than that of a frequency bandwidth. Since data at the low data rate is transmitted through the frequency bandwidth at the high data rate, codes used for distinguishing the subscribers from one another have the properties of the spreading codes. That is, data bit streams at the low data rate are spread by using the spreading codes at the high data rate, and are then transmitted or received through a given frequency bandwidth. As a kind of this CDMA system, a DS-CDMA communication system spreads the data bit streams to be communicated by multiplying the data bit streams by the PN sequences having a code rate higher than that of the data, with the different PN sequences being allocated to the respective subscribers.
Moreover, CDMA mobile communication systems employ a spreading technique for spreading data by using Walsh codes that are spreading codes used for channel separation and spectrum spreading. This spreading technique distinguishes (or separates) the difference subscribers or channels by using the orthogonality of the Walsh codes, without interference among the channels (subscribers). Here, an OQPSK modulation utilizes an offset to maintain the orthogonality.
In one design of an OQPSK modulation device for a DS-CDMA mobile communication system, one finite impulse response filter (i.e., FIR) filters the spread signal XI(t) to generate an I-channel transmission signal SI(t) and another an FIR filter filters the spread signal XQ(t) that has been delayed by one-half of a chip in a xc2xd chip delay in order to generate a Q-channel transmission signal SQ(t). This design endeavors to prevent zero-crossing of the spread signals XI(t) and XQ(t) by phase-delaying the spread signal XQ(t) by 90xc2x0. Therefore, when the FIR-filtered transmission signals SI(t) and SQ(t) are applied to a linear circuit in a rear stage such as a power amplifier, an increase in sidelobe of the transmission signals can be prevented. We have found that unlike a QPSK DS-CDMA communication system not providing an offset, the OQPSK DS-CDMA communication system can not maintain the orthogonality between the I-channel signal XI(t) and the Q-channel signal XQ(t); this results in the occurrence of phase errors. That is, we have noticed that when no channel noise exists during demodulation at the receiver, the orthogonality between the I-channel and the Q-channel is not maintained; this causes phase errors due to mutual interference and thus deterioration of the performance of the system.
It is therefore an object of the present invention to provide an improved process and circuit for generating spread spectrum signals.
It is another object to provide a process and circuit for generating spread signals using orthogonal codes in a DS-CDMA mobile communication system.
It is still another object to provide a code division multiple access process and circuit able to maintain orthogonality between the I-channel and the Q-channel.
It is yet another object to provide a process and circuit for generating spread signals using orthogonal codes in a DS-CDMA mobile communication system, which can minimize phase errors and maintain an orthogonality between an I-channel signal and a Q-channel signal.
It is still yet another object to provide a process and circuit for generating spread signals using orthogonal codes in an offset quadrature phase shift keying modulated circuit for a direct sequence-code division multiple access communication system amenable to avoidance of zero-crossing.
It is a further object to provide a process and circuit for generating spread signals using orthogonal codes in an offset quadrature phase shift keying modulated circuit for a direct sequence-code division multiple access communication system while minimizing any change in sidelobe by avoiding zero-crossing.
These and other objects may be attained in the practice of the instant invention with a process and circuit for generating spread spectrum signals using orthogonal codes in a CDMA mobile communication system. The device includes a zero-crossing detector for detecting an I-channel signal and a Q-channel signal to generate a zero-crossing detection signal, a delay for phase-delaying the Q-channel signal by 90xc2x0, and a multiplexer for outputting one of the Q-channel signal and the phase-delayed signal according to the zero-crossing detection signal. The multiplexer outputs the Q-channel signal to maintain an orthogonality between the I-channel signal and the Q-channel signal, when the zero-crossing is not detected. When the zero-crossing is detected however, the multiplexer phase-delays the Q-channel signal by 90xc2x0 and outputs the phase-delayed Q-channel signal to maintain an one-half chip offset.